Check if NFS mount is active before serving

Hello.

Anyone has idea how to check if NFS drive is mounted and alive before serve files from it ?

If drive is not alive or not mounted, i want to use copy of the files from NFS mount which is cached with cachefs, but for that i need to change my root directory dynamic based on NFS status.

Edit: May be some analogue in nginx of PHP function is_writable() ?

Thanks in advance